1. Field Of the Invention
This invention relates to motion compensated image processing.
2. Description of the Prior Art
Motion compensated image processing is used in image processing applications such as television standards conversion, film standards conversion or video to film conversion. An example of a previously proposed motion compensated image processing apparatus is described in the British Published Patent Application number GB-A-2 231 749, in which pairs of temporally adjacent input images (fields or frames) of a digital video signal are processed to generate corresponding sets of motion vectors for use in motion compensated interpolation of an output image.
The set of motion vectors generated from a pair of input images comprises a plurality of motion vectors for each picture element (pixel) in the output image, albeit that each pixel may have the same plurality of motion vectors as other pixels in a block of the output image.
For each pixel in the output image, the set of motion vectors is supplied to a motion vector selector which tests the plurality of motion vectors corresponding to that pixel in order to select one motion vector for use in interpolation of that pixel. The selected motion vector for each pixel is supplied to a motion compensated interpolator which interpolates the output image from the pair of input images, taking into account the motion between the input images.
In some known motion compensated image processing systems each of the plurality of motion vectors corresponding to a particular output pixel to be tested by comparing test blocks of pixels in the two input images pointed to by that motion vector. In particular, the sum of the absolute luminance differences between corresponding pixels in the two blocks is detected, and the motion vector in the plurality for which this sum has the lowest value is selected for use in interpolation of the output pixel. However, if one or both of the test blocks falls partially outside its respective input image for a particular motion vector, then some of the pixels from which the sum of absolute luminance differences is calculated will be those outside the active picture area of the input frames. These could be pixels in the blanking area of the input frames or could simply be spurious data values generated because the addresses of those pixels were not valid. The result is that the sum of absolute differences calculated using those pixels cannot be used in the vector selection process, and in fact one previously proposed solution to this problem would be to assign the zero motion vector to the output pixel under these circumstances.
An alternative, unsatisfactory, solution would be to mask the outer regions of the output images to conceal errors in motion vector selection.